This invention relates to a mechanism for positioning objects, particularly mirrors, in up to six degrees-of-freedom, namely X-, Y- and Z-translation and rotation about X-, Y- and Z-axes.
Known six degrees-of-freedom mechanisms are varied in structure and operation, resulting in different levels of orientational freedom (work volume), positional repeatability, stiffness and coupling. Coupling is related to the extent to which isolation of motion in one of the degrees-of-freedom is possible.
U.S. Pat. No. 5,028,180 discloses a six degrees-of-freedom motion mechanism that is similar to a Stewart Platform in operation, but is intended for machine tools. Six legs, adjustable in length, connect a platform to a base and can be adjusted to set the platform at a desired orientation.
U.S. Pat. No. 5,263,382 discloses a mechanism that provides six degrees-of-freedom with only three fixed-length legs attached to the movable platform. The legs are in two sections with a one degree-of-freedom hinge joint at their connection point. Each leg is driven by a pair of motors, by way of a differential drive system. The pitch and yaw of each leg is controlled. By controlling the two degrees-of-freedom of each of the three legs, six degrees-of-freedom motion of the moveable platform is accomplished. This mechanism is complex, particularly the differential drive system.
U.S. Pat. No. 5,301,566 discloses a hybrid manipulator. Three fixed length limbs are attached to a platform via universal joints. By changing the location of the lower end of each of the three limbs using two degrees-of-freedom parallel drivers, the platform can be positioned in six degrees-of-freedom.
U.S. Pat. No. 5,333,514 discloses a six degrees-of-freedom parallel manipulator. This manipulator is fully parallel but instead of using linear actuators for the links as in a traditional Stewart Platform, the six arms are in two sections. The first section is mounted to a rotary actuator. By rotating these six base links, the position of the platform can be controlled in six degrees-of-freedom.
U.S. Pat. No. 5,656,905 discloses a hybrid manipulator for machine tools. Two parallel mechanisms are described in which each is a three degrees-of-freedom mechanism. These two mechanisms can be combined in serial fashion to form a hybrid mechanism or they can be combined in parallel to form a cooperating mechanism. One mechanism is dedicated to translational motion while the other is dedicated to rotational motion.
U.S. Pat. No. 5,901,936 discloses a fully parallel six degrees-of-freedom motion mechanism. This mechanism is similar to a Stewart Platform. Disclosed is the use of rotary actuators in place of linear actuators for the legs of the manipulator. There are two main embodiments disclosed. In the first embodiment, there are two fixed length links joined by a hinge at their attachment point. The lower link attaches to the base and the upper link attaches to the moving platform. The hinge joint at the connection of the two links is actuated. Combining six copies of this mechanism allows motion in six degrees-of-freedom. In the second embodiment, a universal driver is used at the base. One axis of the universal joint is actuated while the other is passive. The link then attaches to the moving platform.
U.S. Pat. No. 6,047,610 discloses a hybrid six degrees-of-freedom manipulator. The disclosed device uses two five-bar linkages mounted so that the plane in which they act can rotate. These five-bars are attached to a platform. Coupling the two five-bars together provides five degrees-of-freedom motion. A final motor near the platform provides the last degree-of-freedom. The two serial five-bar linkages, together in parallel arrangement, form the hybrid manipulator.
The mechanisms in the above known devices are fundamentally different from the mechanism providing motion in six degrees-of-freedom in the apparatus of the present invention.
In one aspect, the present invention provides a positioner for transmitting movement in up to six degrees-of-freedom to an object on the positioner, the positioner comprising a base plate, a first plate mounted on the base plate by a first mechanical linkage, a first actuator arrangement for moving the first plate, a second plate mounted on the first plate by a second mechanical linkage, a second actuator arrangement for moving the second plate, and a holder attached to the second plate for mounting the object. One of the first and second plates is movable in a plane in up to three degrees-of-freedom, and the other of the first and second plates is movable in up to three degrees-of-freedom that are out of the plane.
An additional plate, including an actuator arrangement for moving the additional plate, can be mounted by a mechanical linkage between the second plate and the holder and, by this approach, the three degrees-of-freedom systems can be stacked to provide redundantly actuated mechanisms depending on the needs of a given situation. The additional plate can be movable in a plane in up to three degrees-of-freedom if it is desired to provide planar motion, decoupled from non-planar motion, directly to the object in the holder. It is also possible to provide this decoupled motion by arranging the two plates mentioned above with the second plate being movable in a plane in up to three degrees-of-freedom, and the first plate being movable in up to three degrees-of-freedom that are out of the plane in which the second plate can be moved. Such arrangements are suitable when decoupled planar motion of the object is required.
In a preferred embodiment, the first plate is movable in a plane in up to three degrees-of-freedom, and the second plate is movable in up to three degrees-of-freedom that are out of the plane in which the first plate can be moved.
Conveniently, the positioner further includes a first bias for biasing the first plate (also referred to herein as xe2x80x9cstage onexe2x80x9d) against the first actuator arrangement, and a second bias for biasing the second plate (also referred to herein as xe2x80x9cstage twoxe2x80x9d) against the second actuator arrangement. Preferably, the first bias comprises a set of springs (also referred to herein as xe2x80x9cactuator preload springsxe2x80x9d) anchored to the base plate and to the first plate. More preferably, the first bias comprises a first set of springs anchored to the base plate and to the first plate, for biasing the first edge of the first plate against the first actuator, and a second set of springs anchored to the base plate and to the first plate, for biasing the second edge of the first plate against the second and third actuators. Preferably, the second bias comprises a third set of springs anchored to the second plate and to the first plate.
It is also possible to use magnetic-based biasing as the first and/or second bias.
The first actuator arrangement for moving the first plate is suitably a first plurality of actuators mounted on the base plate. Preferably, the first plurality of actuators comprises a first, a second and a third actuator (also referred to herein as xe2x80x9cX-, Y1- and Y2-actuatorsxe2x80x9d, respectively), the first actuator conveniently contacting a first edge of the first plate and the second and third actuators contacting a second edge of the first plate. The first and second edges of the first plate are preferably at substantially right angles to each other (orthogonal). Conveniently, the first, second and third actuators each comprise a micrometer. Motorized micrometers or piezo actuators could also be used.
The second actuator arrangement is suitably a second plurality of actuators mounted on the first plate. Preferably, the second plurality of actuators comprises a fourth, a fifth and a sixth actuator (also referred to herein as xe2x80x9cZ1-, Z2- and Z3-actuatorsxe2x80x9d). More preferably, the fourth, fifth and sixth actuators are spaced apart and extend substantially orthogonally from the first plate and contact a surface of the second plate. Conveniently, the fourth, fifth and sixth actuators each comprise a micrometer assembly that includes a contact pin for contacting the second plate. Motorized micrometers or piezo actuators could also be used.
Conveniently, the first lockable mechanical linkage comprises a plurality of bolts and a corresponding plurality of nuts, preferably three nut and bolt combinations, wherein the bolts are spaced apart and extend through corresponding clearance holes in the base plate and holes in the first plate. A cam lock system could be used in place of a bolt.
The second lockable mechanical linkage suitably comprises three rods, each housed in its own passage in the first plate perpendicular to, and intersecting with, the contact pins (also referred to herein as xe2x80x9cZ-pinsxe2x80x9d) of the fourth, fifth and sixth actuators, respectively. Each rod (also referred to herein as xe2x80x9cZ-pin lock shaftxe2x80x9d) has an indent that partially surrounds a respective contact pin. In addition, each rod has a threaded portion having a nut for tightening to produce frictional engagement of the rod with its respective contact pin. A cam lock system can also be used to produce the frictional engagement.
The holder conveniently comprises a third plate (also referred to herein as xe2x80x9ctransition platexe2x80x9d) substantially parallel to the second plate and connected thereto by a third mechanical linkage, and fourth mechanical linkage for connecting the object to the third plate. The holder may alternatively consist of a regular array of threaded holes in the second plate to allow for custom mounting solutions for varying objects.
The third mechanical linkage suitably comprises a plurality of tooling balls, preferably three, each tooling ball being on a corresponding arm that extends from the third plate. The arms are spaced apart and the balls are sized to pass through corresponding holes in a lock plate into corresponding vee grooves in the second plate. Each hole in the lock plate includes an elongated slot sized to receive a corresponding arm when the lock plate is rotated to a locking position after the tooling balls have been passed through the corresponding holes in the lock plate to lock the third plate to the second plate.
If the object is a mirror, the fourth mechanical linkage conveniently comprises a plurality of flexures, preferably three, secured to the object in a spaced apart configuration, each flexure including an arm extending from the object and terminating in a locking portion that is substantially orthogonal to the arm. The locking portion is sized to pass through a corresponding hole in the third plate, and each corresponding hole in the third plate includes a slot sized to receive an arm when the object is rotated to a locking position after the locking portions have been passed through the corresponding holes in the third plate. Preferably, the locking portion is bolted to the third plate when each arm is in its corresponding slot.
The mechanism of the present invention is a hybrid design that provides motion in six degrees-of-freedom by arranging two, distinct, three degrees-of-freedom mechanisms in series. Additional copies of the three degrees-of-freedom parallel mechanisms can be added in series to increase the degree of control of positioning by the system. The division of the degrees-of-freedom allows the mechanism to be split into two parts, each of which is highly symmetric and has a very low profile. Combining the two, three degrees-of-freedom mechanisms together results in a positioner that is very small and is easily adjustable.
While known parallel mechanisms generally have low work volumes, high mechanical stiffness and complex forward kinematic solutions, the present invention allows high mechanical stiffness to be retained while providing simple kinematics. The positioner of the present invention is useful for positioning mirrors, particularly off-axis conic mirrors, which generally do not require a large work volume.
Preferably, the object on the positioner is a mirror, such as an off-axis conic mirror. Other objects requiring precision positioning and orientation, including lasers etc., are also suitable.
The kinematic solution for the positioner of the present invention can be easily found once the forward and inverse solutions for each of the three degrees-of-freedom parallel mechanisms are known. Closed form solutions for each of the three degrees-of-freedom parallel mechanisms exist and are determinable.
All of the moving components that determine the position of the object are perfectly constrained kinematically. In other words, each moving component is supported at six points in such a way that these six contacts exactly determine the position of the component in space. For a given position of the six contacts, there is exactly one possible position of the component. This makes adjustments extremely repeatable. If the position of each contact is known exactly, then the position of the components will be known exactly.
The motions in six degrees-of-freedom are separated into two distinct mechanisms. Each of these mechanisms has a low height compared to its width and the footprint of the mechanism is similar in size to the object which is being positioned. However, the overall height of the mechanism is approximately just three times that of the object, if the object is a mirror. Compared to commercially available solutions for providing motion in six degrees-of-freedom, this is very small. This small size is important, as many situations require the overall volume of adjustment mechanisms to be minimized.
In applications that require very high accuracy, high stiffness is almost always a requirement. As mentioned earlier, parallel mechanisms tend to be very stiff as the payload is supported at a number of points. Combining a number of single degree-of-freedom mechanisms together to form a six degrees-of-freedom mechanism results in a device with low stiffness in comparison to the present invention. This high stiffness allows the mechanism to be moved under changing conditions such as a gravity vector while maintaining the position of the object very accurately. Once adjustment of the object is complete, its position can be locked which significantly increases its stiffness.
Once the final position of the object is reached in the present invention, the mechanism can be locked to prevent accidental adjustment. While this is not necessary, this is of particular benefit to applications where a one-time alignment will be followed by an operational time of days, weeks or years. The mechanism that is used to lock the adjustments in place is separate from the adjustment mechanisms themselves. This has at least three benefits. Accidental adjustment of the device is near impossible since the locking mechanism will mechanically prevent the actuators from moving the mechanism. The actuators themselves can be removed from the device once the final position is reached. This can allow more accurate and generally more expensive actuators to be used without requiring them to be dedicated to the device. They could be removed from the device after adjustment and used elsewhere, as they are not required to hold the final position of the mechanism. Finally, since the locking mechanism is external to the adjustment mechanism, it can be designed to provide extra stiffness.
The present invention also allows for the repeatable removal and replacement of the object without requiring re-adjustment of the mechanism. It is possible that once the object is positioned correctly there will never be a desire to re-adjust it. However, there is often a requirement to remove the object from the positioner for servicing or upgrading. The present invention allows the object to be removed from the positioner, then serviced and replaced on the device in exactly the same position from which it was removed. This operation is accomplished using a kinematic clamp.
The hybrid nature of the mechanism of the present invention allows for simple workspace optimization for any task. The first three degrees-of-freedom parallel mechanism is responsible for planar translation and rotation (in a Right Hand Coordinate system X translation, Y translation and rotation about the Z-axis). The second, three degrees-of-freedom mechanism is responsible for the remaining two rotations and translation along the Z-axis. It is conceivable that there are problems in which large X and Y translations but little rotation about these axes will be required. Optimizing the design to accomplish this is very simple and has no effect on the accuracy or resolution of the system.
The present invention is also scalable, and can be applied to positioning objects of many sizes. A typical use is for mounting mirrors 100 mm in diameter to 300 mm in diameter. The design, with appropriate considerations made regarding scaling, could be used for mirrors a few centimeters in diameter, or smaller, to meters in diameter. The dimensions of the plates can be adjusted in length, breadth and/or height as required and ultimately the plates may take on any suitable shape that may or may not be plate-like.
The actuators in the present invention are conveniently located in two closely spaced parallel planes. Preferably, three actuators control Z translation, as well as X and Y rotation, and are all located in a plane parallel to the surface of the first plate. Preferably, three actuators control X and Y translation, as well as Z rotation, and are located in another parallel plane between the base and first plate. This arrangement allows for easy access to the actuators from the back of the device. In a preferred embodiment, the X, Y1 and Y2 actuators are stationary, and the Z1, Z2 and Z3 actuators move with the position dictated by these first three actuators.
The six degrees-of-freedom in this device are partially decoupled to allow simple and comprehensible adjustment. To perform accurate adjustments of the object in an arbitrary coordinate system, a computer program can be used to calculate necessary actuator adjustments for a particular final position, as all six degrees-of-freedom become coupled. In other words, the design is amenable to implementation in an automatic control system. By implementing sensors to measure the position of the object and motorizing the actuator arrangements, a closed loop control system can be produced. The algorithms for the software control of the six degrees-of-freedom positioner can be implemented in this control system.
As well as using a computer to calculate the adjustments necessary to position the device, a computer can be used to automate the positioning itself, through communication with motorized actuator arrangements.
The preferred layout of the actuators does allow for decoupled motions in some cases. Also, it is probable that, at some point during an alignment or adjustment procedure, motions may be required based on trial and error. A typical Stewart Platform, or a variation thereof, precludes this type of adjustment as there is no simple way of knowing which actuator motion, or combination of actuator motions, will accomplish the desired action. The present invention is designed in such a way that trial and error adjustments are easily possible, as moving a particular actuator will have the expected effect within some relatively small error margin.